And the Galaxy Said, Let There Be Light

WASHINGTON, DC--In the universe's early days, there wasn't much to see. In fact, there was nothing to see--it was too hot for light to shine. So how did light first appear in the infant universe? A glimpse of radiation leaking out of a small galaxy 281 million light years from Earth has given astronomers a clue.

Right after the Big Bang--roughly 13.7 billion years ago--the universe was so hot that particles of matter were completely ionized, or positively charged. The extreme ionization meant dense structures such as stars and galaxies couldn't form easily. As the universe expanded, it cooled and deionized, thanks to bits of atomic nuclei and electrons coming together to make neutral, light-weight atoms such as hydrogen and helium. Stars began to form, yet they hardly twinkled because the deionized nature of the universe made it difficult for light to travel. Then, between 12.5 billion and 13 billion years ago, the universe became ionized again, and stars really began to shine.

Many astronomers credit the very first stars with driving the reionization. These stars were not locked within galaxies, so they could thus spew their energetic radiation freely throughout the universe. But when astronomer Nils Bergvall of Uppsala Astronomical Observatory in Sweden and colleagues examined a near-Earth galaxy that resembles those that existed in the universe's first billion years, they found another potential driver.

The new source is the birth of stars inside a galaxy. Observations using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite indicated that between 4 and 10% of the ionizing radiation produced by a burst of star formation within the galaxy was able to escape its confines. Previous studies have looked at "echoes" of this leak, says Bergvall, but it's never been measured directly. This finding raises the possibility that lots of tiny galaxies may have together leaked out enough radiation to play a significant role in reionizing the universe, the team reported 12 January here at a meeting of the American Astronomical Society.

This "is a step in the right direction" for nailing down the factors that led to the universe's first light, says cosmologist Abraham Loeb of Harvard University. But ultimately, he says, researchers "want to look at the actual early galaxies not just those that are similar to them." Until there's a telescope powerful enough to peer into the far reaches of the universe, that observation will have to wait.